Log in to the Horizon platform at https://www.unlimitedhorizon.co.uk/webapp/signin
Note that you must be logged in as the Administrator NOT a User.
You will see a screen similar to the one below.
Hover over 'Users' and click 'List Users'.
Click 'Edit' against the User whose Password you wish to reset.
Click 'Reset Password' at the bottom of the screen. This will generate an email to the selected User giving them a new Password to use.